The present invention relates to a method for producing a nano-porous coating on a substrate. In particular, the invention provides a method that is capable of mass-producing coatings for sensor, membrane, and electrode applications.
Porous solids have been utilized in a wide range of applications, including membranes, catalysts, sensor, energy storage (electrodes), photonic crystals, microelectronic device substrate, absorbents, light-weight structural materials, and thermal, acoustical and electrical insulators. These solid materials are usually classified according to their predominant pore sizes: (i) micro-porous solids, with pore sizes  less than 1.0 nm; (ii) macro-porous solids, with pore sizes exceeding 50 nm (normally up to 500 xcexcm); and (iii) meso-porous solids, with pore sizes intermediate between 1.0 and 50 nm. The term xe2x80x9cnano-porous solidxe2x80x9d means a solid that contains essentially nanometer-scaled pores (1-1,000 xcexcm) and, therefore, covers xe2x80x9cmeso-porous solidsxe2x80x9d and the lower-end of xe2x80x9cmacro-porous solidsxe2x80x9d.
A number of methods have previously been used to fabricate macro- or meso-porous inorganic films. Meso-porous solids can be obtained by using surfactant arrays or emulsion droplets as templates. Latex spheres or block copolymers can be used to create silica structures with pore sizes ranging from 5 nm to 1 xcexcm. Nano-porous silica films also can be prepared using a mixture of a solvent and a silica precursor, which is deposited on a substrate. When forming such nano-porous films by spin-coating, the film coating is typically catalyzed with an acid or base catalyst and additional water to cause polymerization or gelation and to yield sufficient strength so that the film does not shrink significantly during drying.
Another method for providing nano-porous silica films was based on the concept that film thickness and density (porosity, or dielectric constant) can be independently controlled by using a mixture of two solvents with dramatically different volatility. The more volatile solvent evaporates during and immediately after precursor deposition. The silica precursor, e.g., partially hydrolyzed and condensed oligomers of tetraethoxysilane (TEOS), is applied to a suitable substrate and polymerized by chemical and/or thermal methods until it forms a gel. The second solvent, called the Pore Control Solvent (PCS) is usually then removed by increasing the temperature until the film is dry. The density or porosity of the final film is governed by the volume ratio of low volatility solvent to silica. It has been found difficult to provide a nano-porous silica film having sufficiently optimized mechanical properties, together with a relatively even distribution of material density throughout the thickness of the film.
Still another method for producing nano-porous inorganic materials is by following the sol-gel techniques, whereby a sol, which is a colloidal suspension of solid particles in a liquid, transforms into a gel due to growth and interconnection of the solid particles. Continued reactions within the sol will lead to a critical chemical state in which one or more molecules within the sol eventually reach macroscopic dimensions so that they form a solid network which extends substantially throughout the sol. At this chemical state, called the gel point, the material begins to become a gel. Hence, a gel may be defined as a substance that contains a continuous solid skeleton enclosing a continuous liquid phase. As the skeleton is porous, the term xe2x80x9cgelxe2x80x9d as used herein means an open-pored solid structure enclosing a pore fluid. Removal of the pore fluid leaves behind empty pores.
The following publications represent the state-of-the-art of the methods for the preparation of nano-porous films or coatings:
1. O. D. Velev, et al.xe2x80x9cPorous silica via colloidal crystallization,xe2x80x9d Nature, 389 (Oct. 1997) 447-448.
2. K. M. Kulinowsky, et al. xe2x80x9cPorous metals from colloidal templates,xe2x80x9d Advanced Materials, 12 (2000) 833.
3. P. R. Coronado, et al., xe2x80x9cMethod for rapidly producing micro-porous and meso-porous materials,xe2x80x9d U.S. Pat. No. 5,686,031 (Nov. 11, 1997).
4. S. C. Jha, et al., xe2x80x9cComposite porous media,xe2x80x9d U.S. Pat. No. 6,080,219 (Jun. 27, 2000).
5. M. Moskovits, et al. xe2x80x9cNanoelectric devices,xe2x80x9d U.S. Pat. No. 5,581,091 (Dec. 3, 1996).
6. R. L. Bedard, et al., xe2x80x9cSemiconductor device containing a semiconducting crystalline nanoporous material,xe2x80x9d U.S. Pat. No. 5,594,263 (Jan. 14, 1997).
7. D. L. Gin, et al., xe2x80x9cHighly ordered nanocomposites via a monomer self-assembly in situ condensation approach,xe2x80x9d U.S. Pat. No. 5,849,215 (Dec. 15, 1998).
8. T. J. Pinnavaia, et al. xe2x80x9cPorous inorganic oxide materials prepared by non-ionic surfactant templating route,xe2x80x9d U.S. Pat. No. 5,622,684 (Apr. 22, 1997).
9. C. J. Brinker, et al., xe2x80x9cMethod for making surfactant-templated, high-porosity thin films,xe2x80x9d U.S. Pat. No. 6,270,846 (Aug. 7, 2001).
10. P. J. Bruinsma, et al., xe2x80x9cMesoporous-silica films, fibers, and powders by evaporation,xe2x80x9d U.S. Pat. No. 5,922,299 (Jul. 13, 1999).
11. R. Leung, et al., xe2x80x9cNanoporous material fabricated using a dissolvable reagent,xe2x80x9d U.S. Pat. No. 6,214,746 (Apr. 10, 2001).
12. R. Leung, et al., xe2x80x9cLow dielectric constant porous films,xe2x80x9d U.S. Pat. No. 6,204,202 (Mar. 20, 2001).
13. K. Lau, et al., xe2x80x9cNanoporous material fabricated using polymeric template strands,xe2x80x9d U.S. Pat. No. 6,156,812 (Dec. 5, 2000).
14. S. K. Gordeev, et al., xe2x80x9cMethod of producing a composite, more precisely nanoporous body and a nanoporous body produced thereby,xe2x80x9d U.S. Pat. No. 6,083,614 (Jul. 4, 2000).
Despite the availability of previous methods for preparing nano-porous silica films, an urgent need exists for a more general method capable of producing a greater variety of metal compounds and ceramic materials in a thin film or coating form. Furthermore, most of the prior art techniques for the preparation of porous coatings are slow and tedious and, hence, not amenable to mass production.
The present invention has been made in consideration of these problems in the related prior arts, and its object is to provide a cost-effective method for directly forming a nano-porous coating onto a solid substrate. In order to produce a uniform, thin, and nano-porous metal compound or ceramic coating on a substrate, it is essential to produce depositable clusters that are on the nanometer scale prior to striking the substrate. These clusters must be capable of partially adhering to each other through parting sintering, liquid bonding, and/or vapor bonding between clusters.
In one embodiment of the present invention, a method entails producing ultra-fine clusters of metal compound or ceramic species and directing these clusters to impinge upon a substrate, permitting these clusters to become solidified thereon to form a thin coating layer. These nano clusters are produced by operating a twin-wire arc nozzle in a chamber to produce metal vapor clusters and by introducing a reactive gas (e.g., oxygen) into the chamber to react with the metal clusters, thereby converting these metal clusters into nanometer-sized ceramic (e.g., oxide) clusters. The heat generated by the exothermic oxidation reaction can in turn accelerate the oxidation process and, therefore, make the process self-sustaining or self-propagating. The great amount of heat released can also help to maintain the resulting oxide clusters in the vapor, liquid, and/or high-temperature solid state. Rather than cooling and collecting these clusters to form individual powder particles, these nanometer-sized vapor clusters can be directed to form an ultra-thin oxide coating onto a solid substrate.
A preferred embodiment of the present invention is a method for producing an optically transparent and electrically conductive coating onto a substrate. The method includes three primary steps: (a) operating a twin-wire arc nozzle to provide a stream of nano-sized metal vapor clusters into a coating chamber in which the substrate is disposed; (b) introducing a stream of oxygen-containing gas into this chamber to impinge upon the stream of metal vapor clusters and exothermically react therewith to produce substantially nanometer-sized metal oxide clusters; and (c) directing these metal oxide clusters to deposit onto the substrate for forming the desired coating.
In the first step, the method begins with feeding a pair of metal wires (either a pure metal or metal alloy) into the upper portion of a coating chamber. The respective leading tips of the two wires are first brought to be in physical contact with each other to form a tentative xe2x80x9cshort circuitxe2x80x9d under a high-current condition and, with the presence of a working gas, form an ionized arc. The arc will heat and vaporize the tips to form nano-sized metal clusters. While the wire tips are being consumed by the arc, the wires are continuously or intermittently fed into an arc cell so that the two leading tips are maintained at a relatively constant separation in a working gas environment. An oxygen-containing gas is introduced into the chamber to react with the metal vapor clusters to form metal oxide clusters. In this case, the oxygen-containing gas serves to provide the needed oxygen for initiating and propagating the exothermic oxidation reaction to form the oxide clusters in the liquid or vapor state, which are then deposited onto the substrate to form a thin coating.
The twin-wire arc spray process, originally designed for the purpose of thermal spray coating, can be adapted for providing a continuous stream of metal vapor clusters. This is a low-cost process that is capable of readily heating up the metal wire to a temperature as high as 6,000xc2x0 C. In an electric arc, the metal is rapidly heated to an ultra-high temperature and is vaporized essentially instantaneously. Since the wires can be continuously fed into the arc-forming cell, the arc vaporization is a continuous process, which means a high coating rate.
The presently invented method is applicable to essentially all metallic materials, including pure metals and metal alloys. When high service temperatures are not required, the metal may be selected from the low melting point group consisting of antimony, bismuth, cadmium, cesium, gallium, indium, lead, lithium, rubidium, selenium, tin, and zinc. When a high service temperature is required, a metallic element may be selected from the high-melting refractory group consisting of tungsten, molybdenum, tantalum, hafnium and niobium. Other metals with intermediate melting points such as copper, zinc, aluminum, iron, nickel and cobalt may also be selected. Indium, tin, zinc, and antimony are currently the preferred choices of metal for practicing the present invention for liquid crystal display applications.
Preferably the reactive gas is an oxygen-containing gas, which includes oxygen and, optionally, a predetermined amount of a second gas selected from the group consisting of argon, helium, hydrogen, carbon, nitrogen, chlorine, fluorine, boron, sulfur, phosphorus, selenium, tellurium, arsenic and combinations thereof. Argon and helium are noble gases and can be used as a carrier gas (without involving any chemical reaction) or as a means to regulate the oxidation rate. Other gases may be used to react with the metal clusters to form compound or ceramic phases of hydride, oxide, carbide, nitride, chloride, fluoride, boride, sulfide, phosphide, selenide, telluride, and arsenide in the resulting coating if so desired.
Specifically, if the reactive gas contains oxygen, this reactive gas will rapidly react with the metal clusters to form nanometer-sized ceramic clusters (e.g., oxides). If the reactive gas contains a mixture of two or more reactive gases (e.g., oxygen and nitrogen), the resulting product will contain a mixture of oxide and nitride clusters. If the metal composition is a metal alloy or mixture (e.g., containing both indium and tin elements) and the reactive gas is oxygen, the resulting product will contain ultra-fine indium-tin oxide clusters that can be directed to deposit onto a glass or plastic substrate.
At a high arc temperature, metal clusters are normally capable of initiating a substantially spontaneous reaction with a reactant species (e.g., oxygen). In this case, the reaction heat released is effectively used to sustain the reactions in an already high temperature environment.
Still another preferred embodiment is a system for producing an optically transparent, electrically conductive coating onto a substrate. The system includes
(a) a coating chamber to accommodate the substrate,
(b) a twin-wire electrode device in supplying relation to the coating chamber for supplying nano-scaled clusters of a metal composition therein. The electrode device includes:
(i) two wires made up of this metal composition, with each wire having a leading tip which is continuously or intermittently fed into the coating chamber in such a fashion that the two leading tips are maintained at a desired separation; and
(ii) means for providing electric currents and a working gas flow for creating an ionized arc between the two leading tips for melting and vaporizing the metal composition to generate the nano-scaled metal clusters; ;
(c) gas supply means disposed a distance from the chamber for supplying a reactive gas into the chamber to react with the nano-scaled clusters therein for forming substantially nanometer-sized metal compound or ceramic clusters; and
(d) supporting-conveying means to support and position the substrate into the chamber, permitting the metal compound or ceramic clusters to deposit and form a coating onto the substrate. Preferably, the supporting-conveying means are made to be capable of transferring, intermittently or continuously, a train of substrate glass pieces into the deposition chamber for receiving the depositable oxide clusters and then transferring them out of the chamber once a coating of a desired thickness is deposited on the substrate.
Advantages of the present invention are summarized as follows:
1. A wide variety of metallic elements can be readily converted into nanometer-scaled oxide clusters for deposition onto a glass or plastic substrate. The starting metal materials can be selected from any element in the periodic table that is considered to be metallic. In addition to oxygen, partner gas species may be selected from the group consisting of hydrogen, carbon, nitrogen, chlorine, fluorine, boron, sulfur, phosphorus, selenium, tellurium, arsenic and combinations thereof to help regulate the oxidation rate and, if so desired, form respectively metal hydrides, oxides, carbides, nitrides, chlorides, fluorides, borides, sulfides, phosphide, selenide, telluride, arsenide and combinations thereof. No known prior-art technique is so versatile in terms of readily producing so many different types of ceramic coatings on a substrate.
2. The metal composition can be an alloy of two or more elements which are uniformly dispersed. When broken up into nano-sized clusters, these elements remain uniformly dispersed and are capable of reacting with oxygen to form uniformly mixed ceramic coating, such as indium-tin oxide. No post-fabrication mixing treatment is necessary.
3. The twin wires can be fed into the arc cell at a high rate with their leading tips readily vaporized. This feature makes the method fast and effective and now makes it possible to mass produce transparent and conductive coatings on a substrate cost-effectively.
4. The system needed to carry out the invented method is simple and easy to operate. It does not require the utilization of heavy and expensive equipment such as a laser or vacuum-sputtering unit. In contrast, it is difficult for a method that involves a high vacuum to be a continuous process. The over-all product costs produced by the presently invented vacuum-free method are very low.